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Alzheimer's disease

Abstract

Alzheimer's disease (AD) is a multifactorial illness manifesting as gradual progressive memory loss, culminating in an end-stage of profound cognitive deterioration. While AD pathology is characterized by the presence of beta amyloid (Aß) plaques, clearing Aß deposits from the brain has not proved sufficient to improve cognition. It is thought that the gradual loss of synaptic connections in the telencephalon leads to cognitive impairment. Thus, understanding synaptic function and testing methods to prevent the loss of synapses are the primary directions of current AD research. AD pathology in the primary sensory areas of the cortex (the granular or koniocortex) is typically found only in the most advanced cases. To gain insight into the differences in the synaptic organization of the koniocortex and the eulaminate cortex, we compared the distribution and morphology of geniculocortical and pulvinocortical terminals in tree shrews. Synaptic terminals were labeled using the stereotaxic injection of neuroanatomical tracers or viral vectors, and/or immunohistochemical labeling of the type 2 vesicular glutamate transporter (vGLUT2) and gamma aminobutyric acid (GABA). Transmitted light, confocal and electron microscopy revealed that geniculocortical terminals in layers IVa and IVb of the striate cortex are significantly larger than pulvinocortical terminals in the temporal cortex. Geniculocortical terminals contact nonGABAergic dendritic spines (91%) and the GABAergic dendrites of cortical interneurons (9%), while pulvinocortical terminals only contact nonGABAergic dendritic spines. Geniculocortical terminals often contain small extensions of postsynaptic spines, termed “spinules”, while pulvinocortical terminals do not. Analysis of the postsynaptic targets of geniculocortical terminals revealed that 14% contained a spine apparatus, an organelle related to dendritic spine stability and memory. Our results indicate that the organization of thalamocortical synaptic connections is quite different in the konicortex and eulaminate cortex. Further comparisons of the synaptic organization of the koniocortex and eulaminate cortex may reveal characteristics related to the progression of AD pathology. The brain areas primarily affected by AD are mirrored by the distribution of dendritic spines that contain spine apparati (uniquely identified by the actin-bindng protein synaptopodin) and by glutamatergic terminals that contain zinc ions sequestered by the type 3 zinc transporter (ZnT3). Because zinc assists in the rapid aggregation of Aß, and zinc levels increase and decrease with brain activity, we examined levels of ZnT3 and synaptopodin in the cortex and hippocampus of a transgenic mouse model of AD (TgSwDI; this model expresses neuronally derived human amyloid ß-precursor protein, APP gene, 770 isoform, containing the Swedish K670N/M671L, Dutch E693Q and Iowa D694N mutations, under the control of the mouse thymus cell antigen 1, theta, Thy1, promoter). Using western blot techniques we measured ZnT3 and synaptopodin levels in tissue from mice at 1, 4 and 6 months of age before and after zinc precipitation by intraperitoneal injection of sodium selenite. We found that sodium selenite treatment produced no significant effect on the levels of synaptopodin. However, we did find that ZnT3 levels were higher in TgSwDI mice at 4 months of age in both the cortex and hippocampus when compared to wild type mice (WT) and TgSwDI mice of other ages. Also, when compared by ages, synaptopodin levels were higher in 4-month old WT and TgSwDI animals in both the cortex and hippocampus. These results suggest that zinc may be an important participant in the pathology of AD, but that age-related changes in ZnT3 levels should be considered when evaluating treatments involving the manipulation of zinc levels. Finally, using electron microscopy and immunohistochemical labeling of activated microglia with isolectin b4 (Ib4), we investigate the histopathology of the cortex and hippocampus in TgSwDI mice. Introduction of Dutch/Iowa mutation caused a strong affinity of Aß deposition near brain vasculature. We found that pathology in the brains of TgSwDI mice progresses in the same areal sequence as is seen in human AD patients and other AD mouse models. However, the cortex and hippocampus are largely devoid of neuritic plaques; the abundance of Aß accumulation was observed in and around blood vessel walls surrounded by microglia cells. Our finding suggest the this mouse model of AD is very suitable for investigations of cerebral amyloid angiopathy-related aspects of AD.